1. Field of the Invention
The present invention relates to a process for the epoxidation of alkenes by catalytic oxidation in the liquid phase. It relates in particular to a process for preparing epoxides by oxidation of alkenes in the liquid phase by means of hydrogen peroxide in the presence of a catalyst system based on a transition metal.
2. Description of the Background
Numerous processes for the epoxidation of alkenes are known, and a wide range of different reaction systems or catalyst systems can be used. The epoxidation of alkenes in a homogeneous, liquid phase by means of organic hydroperoxides in the presence of catalysts based on molybdenum, tungsten or vanadium is employed in industry. However, the preparation of epoxides is accompanied by equivalent or even larger amounts of the alcohol corresponding to the hydroperoxide, and the utilization or recirculation of the alcohol greatly restricts the industrial use of such a process.
For this reason, more direct oxidation processes (epoxidation) of alkenes have been developed.
One such more direct oxidation reaction is epoxidation by means of molecular oxygen using silver catalysts. However, this process has been able to be employed successfully only in the case of ethene; it has not been able to be applied analogously to other alkenes of interest (for example propene).
Another process for direct oxidation of alkenes to epoxides is reaction with hydrogen peroxide. This process has been proposed for various epoxidation reactions especially because of the positive properties of the oxidant in respect of significantly reduced environmental pollution. Since the activity of hydrogen peroxide toward alkenes is only low, in some cases totally absent, it is necessary to employ activating agents, usually organic acids such as formic acid, acetic acid, or the like, in organic solvents. These acids form peracids in situ, and the latter act as the actual reactive epoxidizing agent. These processes, too, do not appear particularly successful, largely because it is difficult to obtain the peracids and because of the instability of the epoxides in an acidic medium, which necessitate rather inconvenient process conditions.
Still another method is the oxidation of alkenes by reaction with highly concentrated hydrogen peroxide in a homogeneous, i.e. exclusively organic, liquid phase in the presence of soluble catalyst systems based on elements of Groups 4, 5 and 6 of the Periodic Table (Ti, V, Mo, W) in combination with elements selected from the group consisting of Pb, Sn, As, Sb, Bi, Hg, and the like. Here too, the results of the process do not permit implementation as an industrial process. This is firstly because the reaction proceeds slowly, and secondly the preparation of the catalyst systems, which generally consist of very complicated organic metal compounds and additionally have to be soluble in the organic reaction medium, is complicated and expensive. Furthermore, the use of highly concentrated hydrogen peroxide (>70%) involves considerable safety risks which cannot easily be overcome in an economical manner.
These processes of the prior art clearly show that the oxidation of alkenes by means of hydrogen peroxide is self-contradictory because the best working conditions in respect of the catalyst system and hydrogen peroxide involve an aqueous, acidic medium while the factors of the oxidation reaction itself and the stability of the epoxide are favored in a neutral organic medium. For this reason, further processes for the epoxidation of alkenes using hydrogen peroxide have been developed, in which either an improved catalyst system based on TiO2/SiO2 in an aqueous phase with addition of primary or secondary alcohols (see EP 0 987 259 A1) or a two phase system containing a catalyst comprising tungstic acid, a quaternary ammonium salt and a phosphorous compound (for example DE 30 27 349) is used.
In the case of alkenes whose epoxides are not hydrolysis-labile and in which the olefinic double bond is not sterically hindered (for example, cyclic at least monounsaturated alkenes), the known epoxidation reaction using hydrogen peroxide and a tungsten catalyst is the most economical alternative.
For the epoxidation reaction by means of hydrogen peroxide to proceed sufficiently quickly, a phase transfer catalyst (for example, ALIOUAT® 336 (tricaprylylmethylammonium chloride)) is usually used in the case of very lipophilic alkenes (for example cyclododecene)(Angew. Chem. (1991), 103(12), 1706-9). However, the desired strong acceleration of the epoxidation by means of the phase transfer catalyst leads to the phases being significantly more difficult to separate after the reaction because of emulsion formation; the corresponding settling times increase greatly. In addition, the organic phase usually remains very turbid after the separation. To achieve virtually complete phase separation, it is necessary to use either phase separators having a very large volume or suitable centrifuges.
The increased settling times in this process greatly reduce its attractiveness for continuous, industrial-scale use. In particular, the process can usually not be implemented at all in existing plants because of space problems caused by the need for larger phase separators. The use of centrifuges is of little interest in view of the power costs and the maintenance requirement due to moving parts.
In quite general terms, it can be said that settling times of less than 2 minutes are industrially desirable. On the other hand, if the settling times are more than 4 minutes, an industrial-scale continuous process is difficult to operate economically.
DE 30 27 349 describes a process for the epoxidation of alkenes using hydrogen peroxide, a tungsten compound, a phosphorous compound and a phase transfer catalyst. In this process, solvents such as alkanes or cycloalkanes are absolutely necessary. These solvents are always added to the reaction mixture in relatively large amounts and generally serve either to dissolve a solid, and thus cause it to react, or improve the reaction conditions, for example in order to achieve better heat removal.
However, the dilution of starting materials with non-reactive substances, for example solvents, is undesirable since, firstly, the dilution leads to a decrease in the space-time yield and secondly a further separation operation after the reaction is necessary.